The European Commission took the case due to the levels of trihalomethanes in drinking water - which are used to remove bacteria.
Europe's highest court says drinking water in Ireland exceeds safety levels.
The Court of Justice says the state failed to meet its obligations on directives.
The court found levels of certain chemical compounds in Irish tap water are both damaging to human health and to the environment.
The European Commission took the case due to the levels of trihalomethanes in drinking water - which are used to remove bacteria.
They can also cause stomach problems and skin irritation.
The case dates back as far as 2014 when Irish Water - now Uisce Eireann - had been asked to ensure drinking water met standards.
Today's findings relate to 21 public water supplies and nine private group schemes.
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